It is known that Organic Light Emitting Diode (OLED) devices can form displays and can also be configured to emit white light and, as such have utility in providing, inter alia, computer monitors, mobile phone displays, televisions, and cheap and efficient lighting for homes, buildings, offices and so on.
In order, for example, to provide a white light OLED one possible device configuration is to have red, green and blue light-emissive species appropriately provided within the device to ensure that the total emitted light is suitable for e.g. general lighting purposes. The provision of distinct red, green and blue emitters within a single device, either within a single layer or in a multilayer device, can lead to performance issues and/or to complexities in device manufacture. For example, it is possible to blend different colour emitters together but this can cause reduced emission intensities in certain cases.
In manufacturing electroluminescent devices such as white light emitting devices, multiple electroluminescent layers may be stacked upon one another, whereby each layer comprises an electroluminescent material having an emission spectrum which differs from that of the other layers. For instance, a multilayer device may comprise a red emission layer, a green emission layer and a blue emission layer. However, such multilayer architecture can result in devices which have low efficiency and/or experience quenching. For example, emission from the triplet state of a phosphorescent emitter may be quenched if a nearby fluorescent emitter that emits from a singlet state also possesses a triplet state of lower energy than the triplet level of the phosphorescent emitter.
To provide OLED displays one possible configuration is to have individually addressable pixels comprising red, green and blue light-emissive species. In such displays, the efficiency (measured for example in Cd/A), quantum efficiency, driving voltage, and lifetime are important operational parameters.
United States Patent Application No 2008/0100199A1 (“The US Application”), discloses polymer material comprising a conjugated polymer and a dendrimer, which may allow manufacture of, for example, a white-light emitting device or green display device that offers the benefits of a lower voltage driving requirement when the polymer is used for a light emitting layer in such a device. This US Application discloses a wide range of monomers that could be employed and its examples note that the dendrimer could be physically mixed into a polymer or could be chemical reacted so that the polymer could contain the dendrimer, for example at the end thereof.
Although the invention described in the US Application represents useful materials that advanced the art, it is believed that further improvements may be made. Thus, for example, it is desired to obtain green emitting materials for use in display or white light emitting devices with good lifetimes and high efficiency while using relatively low levels of expensive metals such as iridium.